Recently, submarine oil fields and submarine natural gas fields are actively developed, and these development activities reach even the deep sea and the icy waters. As a result, marine structures installed on the sea for such development become huge in scale, and it is now impossible to construct such a large-scale marine structure in an existent dock.
As methods for constructing a large-scale marine structure which cannot be constructed in an existent dock as mentioned above, the following methods are known:
(1) As shown in FIG. 4, a plurality of barges, for example two barges 12 are landed via a plurality of blocks 10 onto the bottom of a dock 2 from which water is discharged, so that the upper surface levels of the barges 12 become flush with the level of the site 11. A large-scale marine structure 1 which cannot be assembled in the dock 2 is constructed by using the site 11 and the two barges 12 which are landed via the blocks 10 onto the bottom of the dock 2 and of which the upper surface levels are flush with the level of the site 11. Then, water is introduced into the dock 2 from the surrounding waters until the water surface level in the dock 2 becomes flush with the level of the waters, to cause the two barges 12 to float in the dock 2 together with the thus constructed large-scale marine structure 1, and then, the large-scale marine structure 1 on the two barges 12 is towed out from the dock 2 to the waters. In FIG. 4, 3 is a portal crane (hereinafter referred to as the "prior art 1").
(2) As shown in FIG. 5, a marine structure 1' is constructed, in a dock 2 from which water is discharged, up to a state in which an upper block 1" is not as yet mounted thereon. Then, water is introduced into the dock 2 from the surrounding waters until the water surface level in the dock 2 becomes flush with the level of the waters, to cause the marine structure 1' in the state in which the upper block 1" is not as yet mounted thereon to float in the dock 2, and then, the marine structure 1' is towed out from the dock 2 to the waters. The upper block 1" is mounted by means of a floating crane on the waters onto the marine structure 1' in the state in which the upper block 1" is not as yet mounted thereon, and the marine structure 1' and the upper block 1" are welded together to construct a large-scale marine structure 1 which cannot be assembled in the dock 2 (hereinafter referred to as the "prior art 2").
(3) For example, two blocks 1A and 1B for a large-scale marine structure 1, which blocks have a size corresponding to the construction capacity of a dock not shown, are constructed in the dock from which water is discharged. Then, water is introduced into the dock from the surrounding waters until the water surface level in the dock becomes flush with the level of the waters, to cause the two blocks 1A and 1B thus constructed to float in the dock, and then, the two blocks 1A and 1B are towed out from the dock to the waters. The two floating blocks 1A and 1B thus towed out to the waters are assembled on the waters as shown in FIG. 6, so that respective portions to be welded come into contact with each other. Then, the two floating blocks 1A and 1B are welded together on the waters to construct the large-scale marine structure 1 which cannot be assembled in the dock (hereinafter referred to as the "prior art 3").
(4) For example, two blocks 1A and 1B for a large-scale marine structure 1, which blocks have a size corresponding to the construction capacity of a dock not shown, are constructed in the dock from which water is discharged. Then, water is introduced into the dock from the surrounding waters until the water surface level in the dock become flush with the level of the waters, to cause the two blocks 1A and 1B thus constructed to float in the dock, and then, the two blocks 1A and 1B are towed out from the dock to the waters. The two blocks 1A and 1B thus towed out to the waters are positioned above two barges 12 previously sunk at a prescribed position in the waters. Then, the two barges 12 are caused to float to mount each of the two blocks 1A and 1B on each of the two barges 12, and the two blocks 1A and 1B on the respective barges 12 are assembled on the waters as shown in FIG. 7, so that respective portions to be welded come into contact with each other. Then, the two blocks 1A and 1B are welded together on the two barges 12 floating on the waters to construct the large-scale marine structure 1 which cannot be assembled in the dock (hereinafter referred to as the "prior art 4").
The above-mentioned prior art 1 has the following problems:
(1) When causing the two barges 12 to float in the dock 2, and towing out the large-scale marine structure 1 on the barges 12 from the dock 2 to the waters, an end portion of the large-scale marine structure 1 projects out of the barges 12. The large-scale marine structure 1 on the barges 12 may thus tend to incline and become unstable.
(2) Because of the necessity of retaining a plurality of barges 12 in the dock 2 for a long period of time, running costs of the barges 12 become higher.
The above-mentioned prior art 2 has the following problem:
(1) Operations on the waters of mounting and welding the upper block 1" by means of the floating crane onto the marine structure 1' in the state not yet mounted with the upper block 1", are difficult and inefficient.
The above-mentioned prior art 3 has the following problems:
(1) When welding together the two blocks 1A and 1B on the waters, it is difficult to weld the portions of the blocks 1A and 1B under water.
(2) Because the two floating blocks 1A and 1B swing upon welding same on the waters under the effect of waves of the waters, it is difficult to assemble and weld the portions to be welded together at a high accuracy.
The above-mentioned prior art 4 has the following problem:
(1) Because the two barges 12 swing under the effect of waves of the waters upon welding on the waters the two blocks 1A and 1B mounted on the respective barges 12, it is difficult to assemble and weld the portions to be welded together at a high accuracy.
In order to construct a large-scale marine structure without causing problems as mentioned above, it suffices to expand an existent dock to increase the construction capacity thereof, or construct an extra-large dock having a capacity permitting construction of a large-scale marine structure.
However, expansion of a dock is limited to a certain area from considerations of portal cranes and other existent construction facilities. Construction of an extra-large dock fully equipped with portal cranes and other construction facilities requires investment in a huge amount. Furthermore, even expansion of the dock or construction of an extra-large dock does not necessarily ensure a high operating rate and economic merits since no clear prospect is available as to the future demand for large-scale marine structures.
Under such circumstances, there is a strong demand for the development of a method for efficiently and economically constructing with a small amount of equipment investment a large-scale marine structure which cannot be constructed in an existent dock, but such a method has not as yet been proposed.